[1] Wuhan Univ, Sch Phys & Technol, Dept Elect Sci & Technol, Key Lab Artificial Micro & Nano Struct,Minist Edu, Wuhan 430072, Peoples R China
[2] Wuhan Text Univ, Coll Elect & Informat Engn, Wuhan 430073, Peoples R China
[3] Wake Forest Univ, Dept Phys, Ctr Nanotechnol & Mol Mat, Winston Salem, NC 27109 USA
[4] Wuhan Univ, Ctr Elect Microscopy, Sch Phys & Technol, Wuhan 430072, Peoples R China
[5] Wuhan Univ, MOE Key Lab Artifcial Micro & Nano Struct, Wuhan 430072, Peoples R China